If you see this page (along with default Magento logo and styling), the store was switched into Maintenance mode, by maintenance.flag file in root directory. Most likely "Put store on the maintenance mode while installing/upgrading" checkbox was enabled during extension installation in Magento Connect Manager:

When this option is checked, maintenance.flag file is created in the root directory of your store and store switched on the Maintenance mode.

Another possibility to trigger Maintenance mode was to enable Put store on the maintenance mode while backup creation option when scheduling new backup:

Resolution

To turn the maintenance mode off you need to delete the maintenance.flag file via FTP or Filemanager:

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